groupConnectivityNaive.py: add prettifyGraph

groupConnectivityNaive.py

 from sage.graphs.graph import Graph, DiGraph
 
+def prettifyGraph(G_):
+  """Convert graph into directed one and number its edges."""
+
+  E = [ (u, v, i) for i, (u, v, _) in enumerate(G_.edges()) ]
+  return DiGraph([G_.vertices(), E], format='vertices_and_edges')
+
+
 def flowEnumerator(G, group):
   """Enumerate all flows of given graph.